Russia and Iran criticized the U.N. chief’s decision to withdraw Tehran’s invitation to join this week’s peace conference on Syria, as delegates began to arrive in Switzerland on Tuesday for the long-awaited talks that aim to end the Syrian civil war.

A last-minute U.N. invitation for Iran to participate in the so-called Geneva conference threw the entire meeting into doubt, forcing U.N. chief Ban Ki-moon to rescind his offer late Monday under intense U.S. pressure after Syria’s main Western-backed opposition group threatened to boycott.

After Ban withdrew the invitation, the opposition Syrian National Coalition said it would attend the talks aimed at ending Syria’s crippling three-year civil war, which has killed more than 130,000 and uprooted millions. The opposition said the conference should seek to establish a transitional government with full executive powers “in which killers and criminals do not participate.”

That cleared the way for the negotiations to open Wednesday as planned in the Swiss resort city of Montreux, with high-ranking delegations from the United States, Russia and close to 40 other countries attending. Face-to-face negotiations between the Syrian government and its opponents – the first of the uprising – are to start Friday in Geneva.

Expectations for a breakthrough at the conference are low. The front lines of the war have been largely locked in place since March, and despite suffering their enormous losses, neither the government nor the opposition appears desperate enough for a deal to budge from its entrenched position.

It’s also unclear how the opposition Coalition, a weak and fractured umbrella group with almost no sway over the most powerful rebel groups inside Syria, could enforce any agreement reached in Geneva.

Syria has been ruled by President Bashar Assad’s family since 1970, and Iran is Assad’s strongest regional ally. The Islamic Republic has supplied the Syrian government with advisers, money and materiel since the uprising began in March 2011. Iran’s allies, most notably the Lebanese Shiite militant group Hezbollah, have also gone to Syria to help bolster Assad’s forces.

Iran’s role has infuriated Syria’s opposition forces, which accuse Tehran of in essence invading their country.

The controversy over Iran’s participation in the talks highlighted the fundamental differences over Syria between the United States and Russia, which has shielded Assad’s regime from U.N. sanctions and continued to supply it with weapons throughout the civil war.

In Moscow, Russian Foreign Minister Sergey Lavrov said Ban’s decision to rescind Iran’s invitation was a mistake, but that the Kremlin would try to make the Geneva negotiations work.

“There is no catastrophe, we will push for a dialogue between the Syrian parties without any preconditions,” Lavrov told reporters Tuesday.

At the same time, Russia’s top diplomat took a jab at Ban, saying his decision on Iran “hasn’t helped strengthen the U.N. authority,” and that recalling the offer looked “unseemly.”

Lavrov reaffirmed Russia’s stance that the presence of Iran was essential for the success of the talks, saying Tehran’s absence “isn’t going to help strengthen the unity of the world’s Muslims.”

Syria’s civil war has unleashed sectarian hatreds that have rippled across the wider region. It also has developed into a proxy war between Iran, the region’s Shiite Muslim power, and Saudi Arabia, the Sunni heavy weight.

“From our point of view, the withdrawal is deplorable,” ministry spokeswoman Marzieh Afkham said, adding that the U.N. chief only did so under immense pressure.

Afkham said that Iran expects Ban Ki-moon will explain the “real reasons” for withdrawing the invitation.

The insistence on holding the Geneva talks despite the low expectations is a reflection of the near unanimity in the international community to find a political resolution to end the Syrian conflict. That urgency stems in part from the humanitarian crisis the war has unleashed, as well as the destabilizing effect it has had on the entire region.

A car bombing in the Lebanese capital’s southern suburbs early Tuesday provided the latest evidence of Syria’s detrimental effect on its neighbors. The explosion struck a Shiite area that is a bastion of support for Hezbollah, killing at least two people.

Lebanon is deeply divided by the war in Syria, and Lebanese Sunnis and Shiites have lined up behind their brethren in Syria on opposing sides of the war!!